The nickel salicide (self-aligned silicidation) process has been considered as a possible replacement for the traditional titanium salicide process on source/drain and polysilicon gate regions in semiconductors because of the lack of linewidth dependence of nickel silicide. While titanium disilicide increases in sheet resistance with decreasing linewidth, nickel monosilicide has a sheet resistance which gradually decreases with linewidth.
Also, the nickel salicide process has been considered as a possible replacement for the traditional titanium salicide process because of its processing simplicity. While the titanium salicide process involves two rapid thermal anneals at 700.degree. C. and 850.degree. C., the nickel salicide process only needs one rapid thermal anneal at a lower temperature between 400.degree. C. to 600.degree. C.
However, the nickel salicide process has not become a comprehensive replacement because it is subject to process variation not only because nickel is a less reactive metal in contact with silicon oxide, but also because the rapid thermal anneal temperatures for nickel are too low to initiate silicon oxide reduction. This means that the residual surface silicon oxide on the surface of the silicon substrate will interfere with the transition of nickel to nickel silicide during the annealing step. On the other hand, the titanium salicide process is known to be robust partially thanks to the ability of titanium to reduce surface silicon oxide and readily react with the silicon substrate.
As a result of the aforegoing, despite the advantages of nickel, titanium has been the metal of choice for a long time. A solution to these problems has been long sought but elusive to those skilled in the art.